The present invention relates generally to electronic filters, and more particularly, to tunable filters that operate at microwave frequencies at room temperature.
Electrically tunable microwave filters have many applications in microwave systems. These applications include local multipoint distribution service (LMDS), personal communication systems (PCS), frequency hopping radio, satellite communications, and radar systems. There are three main kinds of microwave tunable filters, mechanically, magnetically, and electrically tunable filters. Mechanically tunable filters are usually tuned manually or by using a motor. They suffer from slow tuning speed and large size. A typical magnetically tunable filter is the YIG (Yttrium-Iron-Garnet) filter, which is perhaps the most popular tunable microwave filter, because of its multioctave tuning range, and high selectivity. However, YIG filters have low tuning speed, complex structure, and complex control circuits, and are expensive.
One electronically tunable filter is the diode varactor-tuned filter, which has a high tuning speed, a simple structure, a simple control circuit, and low cost. Since the diode varactor is basically a semiconductor diode, diode varactor-tuned filters can be used in monolithic microwave integrated circuits (MMIC) or microwave integrated circuits. The performance of varactors is defined by the capacitance ratio, Cmax/Cmin, frequency range, and figure of merit, or Q factor at the specified frequency range. The Q factors for semiconductor varactors for frequencies up to 2 GHz are usually very good. However, at frequencies above 2 GHz, the Q factors of these varactors degrade rapidly.
Since the Q factor of semiconductor diode varactors is low at high frequencies (for example,  less than 20 at 20 GHz ), the insertion loss of diode varactor-tuned filters is very high, especially at high frequencies ( greater than 5 GHz ). Another problem associated with diode varactor-tuned filters is their low power handling capability. Since diode varactors are nonlinear devices, larger signals generate harmonics and subharmonics.
Varactors that utilize a thin film ferroelectric ceramic as a voltage tunable element in combination with a superconducting element have been described. For example, U.S. Pat. No. 5,640,042 discloses a thin film ferroelectric varactor having a carrier substrate layer, a high temperature superconducting layer deposited on the substrate, a thin film dielectric deposited on the metallic layer, and a plurality of metallic conductive means disposed on the thin film dielectric, which are placed in electrical contact with RF transmission lines in tuning devices. Another tunable capacitor using a ferroelectric element in combination with a superconducting element is disclosed in U.S. Pat. No. 5,721,194.
Commonly owned U.S. patent application Ser. No. 09/419,126, filed Oct. 15, 1999, and titled xe2x80x9cVoltage Tunable Varactors And Tunable Devices Including Such Varactorsxe2x80x9d, discloses voltage tunable dielectric varactors that operate at room temperature and various devices that include such varactors, and is hereby incorporated by reference.
There is a need for tunable filters that can operate at radio frequencies with reduced intermodulation products and at temperatures above those necessary for superconduction.
This invention provides an electronic filter including a substrate, a ground conductor, an input, an output, a first microstrip line positioned on the substrate and electrically coupled to the input and the output, and a first tunable dielectric varactor electrically connected between the microstrip line and the ground conductor. The input preferably includes a second microstrip line positioned on the substrate and having a portion lying parallel to the first microstrip line. The output preferable includes a third microstrip line positioned on the substrate and having a portion lying parallel to the first microstrip line. The first microstrip line includes a first end and a second end, the first end being open circuited and the varactor being connected between the second end and the ground conductor. The filter further includes a bias voltage circuit for supplying control voltage to the varactor. In the preferred embodiment, the bias circuit includes a high impedance line, a radial stub extending from the high impedance line, and a patch connected to the high impedance line for connection to a DC source. The varactor preferably includes a substrate having a low dielectric constant with a planar surface, a tunable dielectric layer on the planar substrate, with the tunable dielectric layer including a Barium Strontium Titanate composite, and first and second electrodes on the tunable dielectric layer and positioned to form a gap between the first and second electrodes. In a multiple pole embodiment, the filter further includes additional microstrip lines positioned on the filter substrate parallel to the first microstrip line and additional tunable dielectric varactors electrically connected between the additional microstrip lines and the ground conductor.